1. Field of the Invention
The present invention relates to an image processing apparatus for solving a problem of clarity deteriorating when moire is reduced in image transformation processing.
2. Description of the Related Art
It is known that in image processing apparatuses, image transformation processing is performed as necessary. For example, in an image processing apparatus in which a projector product is mounted, image transformation processing called keystone (trapezoid) correction processing is performed. To explain specifically, when output light of the projector is projected onto a screen, for an effective region projected onto the screen a trapezoid shaped distortion occurs due to an installation inclined angle of the projector or an optical lens shift. Because, in a state in which there is a trapezoid shaped distortion in the effective region projected on the screen, it is difficult for a user to see, processing is performed to perform an image transformation so that the effective region projected onto the screen becomes a rectangular shape by performing an image transformation of the effective region for an inverted trapezoid shape. This image transformation processing is generally known as keystone (trapezoid) correction processing.
In order to perform image transformation processing with an object of keystone (trapezoid) correction processing, or the like, after storing, into a frame memory, an input image, each coordinate of an output image is generated by interpolating from pixels in a vicinity of the input image coordinates corresponding to the coordinates of the output image. A method such as, for example, a bi-cubic interpolating method, or the like is generally known as this interpolation method.
Note, there are cases in which image degradation called moire (interference fringes) occurs when image transformation processing is performed. Moire is an interference fringe that occurs due to a difference between a pixel pitch of the output image, and a pitch of coordinates on the input image corresponding to the pixels of the output image. The image degradation due to moire is caused by high-frequency components of the source image, and a method for removing high-frequency components by performing a smoothing process before the image transformation processing in order to ameliorate this is known (Japanese Patent Laid-Open No. 1-134577).
However, when the moire reduction processing (smoothing processing) of Japanese Patent Laid-Open No. 1-134577 is applied, though the moire is reduced, there is an adverse effect in that edges are smoothed and clarity deteriorates. Specifically, there are many cases in which it is desirable that regions such as characters in an image be clear, but as a result of the smoothing processing edges are dulled, and an output image in which the characters are unclear is generated. In response to this, in order to improve Japanese Patent Laid-Open No. 1-134577, a unit for searching for edge regions is used, and by applying weaker smoothing processing to edge regions than to non-edge regions, edge regions of characters or the like becoming unclear is prevented in a technique of Japanese Patent No. 3143209.
The method of Japanese Patent Laid-Open No. 1-134577 is a method for reducing moire by removing high-frequency components by performing smoothing processing before the image transformation processing. However, as previously explained, though the moire is reduced, there is an adverse effect in that edges are smoothed and clarity deteriorates with this method. Explanation will be given for this problem using FIGS. 2A-2D. FIGS. 2A-2D are views for explaining a moire reduction effect due to smoothing processing. Note, in order to simplify the explanation with these views, an image that is periodic in a one-dimensional direction is used as the input image. Explanation will be given for the relationship of the drawings of FIGS. 2A-2D below.
Firstly, FIG. 2A is an input image having a first period of two values (white/black) in a horizontal direction, and for FIGS. 2B-2D various processing (transformation processing, or smoothing processing) has been performed as shown graphically here. FIG. 2B is an image resulting from transformation processing being applied to the input image of FIG. 2A, and a second period not in FIG. 2A appears. The transformation processing is an example of one-dimensional direction enlargement processing, and due to interference of the period of the input image and the magnification factor here, the second period appears. So, periodic moire due to the amplitude of this second period is visibly recognizable. On the other hand, FIG. 2C is an image resulting from smoothing processing being applied to the input image of FIG. 2A, and FIG. 2D is an image resulting from transformation processing being further applied to the image of FIG. 2C. As shown in FIG. 2C, by applying the smoothing processing to the input image of FIG. 2A, a peak of the first period is reduced. Furthermore, as illustrated in FIG. 2D, it is visibly recognizable that the moire due to the transformation processing being applied to the image of FIG. 2C is reduced. In other words, compared to FIG. 2B for which the smoothing processing is not performed, FIG. 2D has the same moire period corresponding to the second period, but the difference of the amplitude of the period shown graphically as the amplitude of the moire becomes smaller, and as a result, it is visibly recognizable that the moire is reduced. However, while the moire is reduced, the amplitude of the first period, corresponding to the original pattern of FIG. 2A before performing the transformation processing, is reduced as well. As a result, this is visibly recognizable as an adverse effect of a deterioration in clarity. To summarize the above described explanation, there is the effect that moire is reduced by the smoothing processing, but there is the adverse effect that the clarity of the input image is reduced.
Meanwhile, in Japanese Patent No. 3143209, in order to improve the above described adverse effect in Japanese Patent Laid-Open No. 1-134577, by applying weaker smoothing processing for edge regions than for non-edge region, edge regions becoming unclear is prevented. However, this method has a problem in that the moire reduction effect is weak. This is because when the smoothing processing on an edge region is weakened, the reduction effect on the moire is weakened, since the edge regions having high frequencies are the cause of the moire, as shown in FIGS. 2A-2D.